Pressure-sensitive adhesives (PSAS) have been known and used for a long time in the art. Some of the common pressure-sensitive adhesives are formulations based upon acrylates, natural rubbers, synthetic rubbers, vinyl acetates, and silicones. The PSAs are typically formulated for end use, often in a solvent or waterborne system. Acrylate PSAs are of particular utility in that they are relatively low in cost, adhere well to a variety of different surfaces, and can be formulated to build adhesion to a surface. However, acrylate PSAs typically have poor high temperature performance and poor low temperature performance. Examples of such acrylate PSAs are disclosed in U.S. Pat. No. Re 24,906 (Ulrich).
In recent times, there has been a need to reduce solvent pollution by using solventless systems in order to prepare pressure-sensitive adhesives. Substantially solvent free acrylate pressure-sensitive adhesives are disclosed in U.S. Pat. No. 4,181,752 (Martens) wherein alkyl acrylate esters and modifying copolymerizable monomers are ultraviolet radiation polymerized to form the acrylate copolymer. The PSAs made by the disclosed process are improved over those made by solution polymerization but are still not completely adequate for some uses such as adhesion at high and low temperature extremes.
Silicone PSAs exhibit the flexibility of silicone rubber and the high temperature stability of silicone resin, thereby providing useful service temperatures ranging between -70.degree. C. and 250.degree. C. Their excellent electrical properties and chemical inertness are noteworthy attributes making them suitable for use in the plating and electronics industry. Silicone PSAs bond to a wide variety of substrates including surfaces having high or low surface energies; e.g., glass, paper, polyolefins, polytetrafluoroethylene, fluorohalocarbon films, and silicone release liners. Most acrylic PSAs will not bond, or adhere poorly, to many of these low energy surfaces, while silicone PSAs are effective bonding agents for these materials. Silicone PSAS, due to their biocompatibility, are widely used in transdermal drug delivery systems and also as general medical adhesives.
Silicone PSAS, however, have a number of disadvantages including the following. First, silicone PSAs are expensive. Second, most silicone PSAs are solvent-based, which limits the thickness at which they may be coated. Third, silicone PSAs are slow to cure due to the need for a solvent removal step prior to crosslinking at temperatures in excess of 130.degree. C. In addition, silicone PSAs generally have low tack and limited adhesion build.
Combining silicone PSAs and acrylate PSAs to achieve specific properties has been attempted. JP 62-295982 (murachi) describes an automotive protective molding having a pressure-sensitive adhesive made by combining a silicone pressure-sensitive adhesive and an acrylate pressure-sensitive adhesive with a polyurethane or polyisocyanate crosslinker. The PSAs are solvent based. The problem addressed by the publication is to obtain a pressure-sensitive adhesive that has high shear strength to resist separation of the adhesive from the molding because of expansion and contraction of the molding due to changes in temperatures. Disadvantages of this adhesive system are a limited pot life in coating the adhesive because of the crosslinker, shelf stability of the coated product which may be affected by the crosslinker, and the need to dry off large amounts of organic solvent since the silicone and acrylic pressure-sensitive adhesives are initially 40% and 37% solids, respectively, and are further diluted with solvent as shown in the examples.
U.S. Pat. No. 4,791,163 (Traver) describes an emulsion pressure-sensitive adhesive comprising from 50 to 99% of an organic pressure-sensitive adhesive and from 1 to 50% of a silicone pressure-sensitive adhesive. The solvent based silicone pressure-sensitive adhesive in solvent is emulsified and the resulting emulsion is added to an acrylic emulsion pressure-sensitive adhesive, thereby providing an emulsified composition, however, said composition contains significant levels of solvent introduced via the silicone PSA. Careful control of the emulsifying agent and the drying conditions is required to prevent phase separation of the emulsion during the drying step. Again, large amounts of energy are required to dry the emulsion to a pressure-sensitive adhesive.
European Patent Publication No. 289928 (General Electric), published Nov. 9, 1988, describes an emulsion or solution comprising: (a) 100 parts by weight of water or organic solvent; (b) from about 10 to about 400 parts by weight of pressure-sensitive adhesive comprising: (i) from about 50 to about 99% by weight organic pressure-sensitive adhesive; and (ii) from 1 to about 50% by weight of silicone pressure-sensitive adhesive; and (c) an effective amount of organic peroxide or alkoxy silane cross-linking agent to increase the shear strength of the composite adhesive through crosslinking of the silicone. The emulsion generally requires the use of an emulsifying agent or agents to maintain both the micelles of silicone adhesive and micelles of organic adhesive in a substantially stable state of suspension even at low water content, so that drying may be accomplished prior to gross phase separation of the silicone adhesive and the organic adhesive.
JP 61-57355 discloses solvent based adhesives having a silicone pressure-sensitive adhesive, an acrylate pressure-sensitive adhesive, and an organic peroxide crosslinking agent to prevent phase separation. This publication discloses that isocyanate crosslinkers are not desirable because it does not prevent phase separation of the acrylic and silicone PSA'S. The examples show that the PSA needs to be cured at 150.degree. C. for 3 minutes.
JP 63-291971 discloses solvent based adhesives that are a mixture of silicone pressure-sensitive adhesive, acrylate pressure-sensitive adhesive, a bridging agent, and a "silicone macromonomer". The adhesives disclosed in this publication have poor shear strength without the "silicone macromonomer." This publication also states that adhesives made as disclosed in JP 61-57355 have poor adhesion.
A need thus exists for a PSA system which has the advantages of both acrylate PSAs and silicone PSAs which requires little or no solvent, thereby reducing or eliminating the environmental and health hazards associated with solvent use, as well as the need for drying. A need also exists for such a PSA system which is curable by an energy efficient means such as radiation and which, unlike most known silicone acrylate systems, is not prone to gross phase separation problems. A need also exists for a silicone/acrylic PSA system which possesses balanced PSA properties tailorable over a wide range, thereby providing greater flexibility than known systems in achieving substrate-specific adhesion.
The inventors of the present invention have discovered an adhesive composition having excellent adhesion to a broad range of substrates over a broad temperature range. The adhesive composition is virtually solvent-free and can be efficiently cured by exposure at ambient temperatures to a number of radiation sources including those capable of emitting ultraviolet radiation, electron beam radiation, actinic radiation, and gamma radiation. Preferably, the adhesive compositions is cured via ultraviolet radiation. Ultraviolet radiation can be provided by various sources having emission spectra between about 250 to about 400 nanometers depending upon the initiators and crosslinkers that are used.
The adhesive compositions of the invention are useful in making pressure-sensitive adhesive transfer tapes or coated adhesive tapes.
The adhesives of the invention exhibit excellent adhesion to the new paints in use in the automobile industry as well as superior resistance to detachment at low temperatures as compared to known acrylic adhesives.
The adhesives of the invention exhibit utility in the attachment of body side molding, decorative trim, weatherstripping, and the like, to automobile surfaces painted with the new paints that are in use in the automotive industry. These new paints are formulated for environmental conservation, enhanced appearance, and durability, such as resistance to degradation from common sources of contamination such as acid rain. The changes in the paint formulations have increased the difficulty with which known acrylate pressure-sensitive adhesives can adhere to substrates coated with such paints. The automobile industry requires that the adhesive, or a tape made with the adhesive, have good adhesion to the paint surface and the molding, and resist detachment at temperatures as low as -40.degree. C. The adhesives of the present invention meet these stringent requirements of the automotive industry.
The adhesives of the invention also show great utility in medical applications because the adhesion build on human skin is sufficient but not excessive, thus facilitating the removal of a bandage coated with the adhesive from human skin.